TUCSON, Ariz.  A study billed by its researchers as the most detailed projection yet of climate change says hotter, drier Southwestern summers will become a reality by the late 21st century if human-caused global warming continues.

The number of extremely hot summer days -- those in the top 5 percent of the 105- to 112-degree range -- could jump 560 percent by late in the century from today, according to the Purdue University study.

The study also says heat waves would last longer, up to 15 days each from northern Mexico into Nevada and Utah. Summer rainfall, which can cause severe flooding but also nourishes rivers, streams and aquifers that provide water to people and wildlife, would fall.

The predicted changes are large enough to substantially disrupt the U.S. economy and its roads, bridges and other public infrastructure, said Noah Diffenbaugh, a Purdue assistant professor who headed the research team behind the study.

Diffenbaugh said the study targets the period 2071-85 and assumes the temperature and rainfall changes would occur gradually, starting before then.

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Two climate specialists at the University of Arizona agree the Purdue study is one of the most thorough of its type, but they say it's still not the last word on the regional effects of global warming.

Jonathan Overpeck, director of the UA's Institute for the Study of Planet Earth, said the study is one of the first to use really good global and regional computer models together to look at possible climate change during the next 90 years.

For several years, scientific researchers have disagreed on whether global warming would mean more or less rain in the Southwest, though they generally agree the weather here will get hotter.

Even if rainfall increases, however, researchers often have warned that the warmer temperatures could cause snowfall to melt more quickly. That would bring rainfall runoff into the region's rivers earlier -- at a time when some reservoirs may not be equipped for it.

The Purdue study predicted little overall, year-round rainfall change across much of the Southwest.